CONTRIBUTIONS OF THE MESODERM TO PRIMITIVE BODY FORMATION 527 



b. Early Differentiation of the Mesomere (Nephrotome) 



The differentiation of the nephrotome or intermediate mesoderm will be 

 considered later (Chap. 18) in connection with the urogenital system. 



c. Early Differentiation and Derivatives of the Hypomere 



The lateral-plate mesoderm (hypomere), figure 252A, performs an ex- 

 tremely important function in embryological development. The cavity of the 

 hypomere (splanchnocoel) and the cellular offspring from the hypomeric meso- 

 derm, which forms the wall of this cavity, give origin to much of the struc- 

 tural material and arrangement of the adult body. 



1) Contributions of the Hypomere (I.ateral Plate Mesoderm) to the De- 

 veloping Pharyngeal Area of the Gut Tube. The developing foregut (Chap. 

 13) may be divided into four main areas, namely, (1) head gut, (2) pha- 

 ryngeal, (3) esophageal, and (4) stomach areas. The head gut is small and 

 represents a pre -oral extension of the gut; the pharyngeal area is large and 

 expansive and forms about half of the forming foregut in the early embryo; 

 the esophageal segment is small and constricted; and the forming stomach 

 region is enlarged. At this point, however, concern is given specifically to the 

 developing foregut in relation to the early development of the pharyngeal 

 region. 



In the pharyngeal area the foregut expands laterally. Beginning at its an- 

 terior end, it sends outward a series of paired, pouch-like diverticula, known 

 as the branchial (pharyngeal or visceral) pouches. These pouches push out- 

 ward toward the ectodermal (epidermal) layer. In doing so, they separate 

 the lateral plate mesoderm which synchronously has divided into columnar 

 masses or cells (fig. 252E, F). Normally, about four to six pairs of branchial 

 (pharyngeal) pouches are formed in gnathostomous vertebrates, although in 

 the cyclostomatous fish, Petromyzon, eight pairs appear. In the embryo of the 

 shark, Squalus acanthias, six pairs are formed, while in the amphibia, four 

 to six pairs of pouches may appear (fig. 252F). In the chick, pig, and human, 

 four pairs of pouches normally occur (figs. 259, 261 ). Also, invaginations or 

 inpushings of the epidermal layer occur, the branchial grooves (visceral fur- 

 rows); the latter meet the entodermal outpocketings (figs. 252F; 262B). 



The end result of all these developmental movements in the branchial area 

 is to produce elongated, dorso-ventral, paired columns of mesodermal cells 

 (figs. 252E; 253), th? visceral or branchial arches, which alternate with the 

 branchial-groove-pouch or gill-slit areas (figs. 252F; 253). The most anterior 

 pair of visceral arches forms the mandibular visceral arches; the second pair 

 forms the hyoid visceral arches; and the succeeding pairs form the branchial 

 (gill) arches (figs. 239C, D; 240; 244; 246; 252E; 253). The branchial arches 

 with their mesodermal columns of cells will, together with the contributions 

 from the neural crest cells referred to above, give origin to the connective, 

 muscle, and blood-vessel-forming tissues in this area. 



